This research project is designed to amplify ongoing studies on the cellular immunological events of acute rejection of vascularized organ allografts and the dynamics of their increased survival in enhanced recipients. Accurate knowledge of the host responses cannot be acheived unless the nature and function of recipient cells infiltrating allografts are identified, their individual function elucidated and their relationships defined. Additionally, such information must be compared and contrasted to those events occurring concomitantly within recipient lymphoid tissues. Infiltrating cell populations will be harvested from heterotopic cardiac allografts, characterized and evaluated functionally. The role of B lymphocytes in acute rejection will be noted by studying host cells infiltrating indefinitely surviving cardiac allografts placed in T deprived (thymectomized, irradiated, bone marrow replaced) rats. Examination of these animals will also allow increased information regarding the multi-faceted role of T cells in grafts. Suppressor activity has been detected in the infiltrating cells taken from enhanced cardiac allografts. The nature of suppression in the enhancement phenomenon will be evaluated fully using several animal models. This multi-faceted investigative approach will allow us to obtain more precise understanding of immunological events initiated by the host against foreign tissues than has been described hitherto. Inbred strains of rats and mice will be used throughout these experiments.